The present invention relates to a fixing apparatus which is for thermally fixing an image on recording medium, and is used for an image forming apparatus, such as a copying machine, a printer, or the like, which employs an electrophotographic recording method, an electrostatic recording method, or the like.
An electrophotographic copying machine or the like is provided with a heating apparatus, which is for fusing a toner image (unfixed image) on a recording medium to the recording medium, by thermally melting the toner (developer) of the toner image while the recording medium, which is bearing the unfixed toner image, is being conveyed.
There are various heating apparatuses, most of which are provided with a fixing roller as a heating medium. It is known that various attempts have been made in order to quickly increase the temperature of the fixing roller. For example, the fixing roller has been reduced in diameter; the wall of the fixing roller has been reduced in thickness; and/or a heating medium placed in the hollow of a rotational cylinder of film has been pressed against the recording medium, through the rotational cylinder of film. Further, in some fixing apparatuses, a thin metallic rotational member is heated by induction. In spite of the difference in approach, the gist of all the attempts has been to reduce the thermal capacity of the rotational member, that is, the heating medium, in order to heat the recording medium with the use of a heat source which is superior in heating efficiency.
Further, there are a few fixing apparatuses which employ a noncontact heat source. However, in consideration of cost and energy efficiency, more contact heating apparatuses have been proposed as a heating apparatus for an image forming apparatus such as a copying machine. In the case of a contact heating apparatus, a rotational member with a thin wall is placed in contact with a recording medium to heat the developer on the recording medium in order to thermally melt the developer.
However, a contact heating apparatus such as the one described above suffers from the following problems: a rotational member with a thin wall employed as a heating medium in order to reduce the thermal capacity of the heating medium is very small in the sectional area, perpendicular to the axial direction of the heating medium, being therefore inferior in the thermal conduction in the direction parallel to the axial direction of the heating medium; the thinner the wall of the heating medium, the worse the above described thermal conduction. Further, the usage of a resinous material, which generally is low in thermal conduction, as the material for the rotational member with a thin wall, makes worse the thermal conduction of the rotational member in the direction parallel to the axial direction of the rotational member.
This is evident from Fourier law of heat conduction, which shows the amount (Q) of heat conducted per unit of time between given two points:
Q=xcexxc2x7f(xcex81xe2x88x92xcex82)/L 
xcex: thermal conductivity or conduction
xcex81-xcex82: temperature difference between two points
L: length
This means that there will be no problem when a recording medium, the dimension of which in terms of the direction parallel to the lengthwise direction of the rotational member, or the heating medium, is the same as the length of the rotational member, is passed through the fixing apparatus for fixation, but that when a plurality of recording mediums, the dimension of which in terms of the direction parallel to the lengthwise direction of the rotational member, is less than the length of the rotational member, are passed in succession, there will be a problem in that the temperature or the portion of the rotational member outside the recording medium path will become higher then the specific value to which the temperature of the rotational member is set for image fixation; in other words, the temperature difference between the portion of the rotational member outside the recording medium path and the portion of the rotational member inside the recording medium path, will become extremely large.
It is possible that this problem, that is, the nonuniformity of the temperature of the heating medium in terms of the lengthwise direction of the heating medium, will reduce the durability of the components in the adjacencies of the heating medium, which are formed of resinous material, and/or will damage the components. Further, it is also possible that this problem will cause a problem that when a recording medium with a larger size is passed through a fixing apparatus structured as described above immediately after a substantial number of recording mediums with a smaller size are passed. The nonuniformity of the temperature of the heating medium in its lengthwise direction will wrinkle and/or skew the larger recording medium, and/or will result in the nonuniform fixation of the image on the larger recording medium.
The higher the throughput (number of prints produced per unit of time), the greater the amount of the temperature difference between the portion of the heating medium outside the recording medium path and the portion of the heating medium inside the recording medium path. This makes it difficult to use a heating apparatus, the heating medium of which is a rotational member with a thin wall and a low thermal capacity, as the fixing apparatus for a copying machine or the like, the throughput of which is relatively high.
There have also been known various heating apparatuses in which a halogen lamp or a heat generating resistor is used as a heat source. Among some of these heating apparatuses, the heat source is divided into a certain number of sections which can be independently activated so that electrical power can be supplied to virtually only the sections of the heat source, the positions of which correspond to the path of the recording medium being passed.
Further, there have been known heating apparatuses, the heat source of which comprises a plurality of discrete induction coils, which can be selectively supplied with electrical power.
However, the provision of a plurality of heat sources, or the division of a heat source into a plurality of sections creates a problem; the greater the number of heat sources or heat source sections, the more complicated the control circuit, and therefore, the more costly. In addition, if an attempt is made to match the number of heat sources, or the number of the sections into which a heat source is divided, with the width of the recording medium path, which varies depending on the recording medium in use, the number of heat sources, or the number of sections into which a heat source is divided, increases, increasing thereby apparatus cost. Further, where a rotational member with a thin wall, which has a given number of sections, is used as a heating medium, it is possible that the temperature distribution across the borders between the adjacent two sections will become discontinuous and nonuniform, affecting the fixing performance.
Thus, various proposals have been made as the solutions to the above described problems. According to some of the proposals, a heating medium is provided with a magnetic flux blocking means, and a moving means for changing the position of the magnetic flux blocking means. The magnetic flux blocking means is for partially blocking the magnetic flux, which is radiated from a magnetic field generating source toward a heating medium. For example, according to the inventions disclosed in Japanese Laid-open patent Applications 9-17889 and 10-74009, a magnetic flux blocking means, and a means for moving the magnetic flux blocking means, are provided to block the magnetic flux from the magnetic flux radiating source, except for the portion of the magnetic flux which is destined to reach the portion of the heating medium necessary to be heated; in other words, the heat distribution of the heating medium is controlled by generating heat only in the portion of the heating medium necessary to be heated for the fixation of an image on the recording medium being passed through the heating apparatus.
In order to prevent the temperature of the magnetic flux blocking plate itself from rising, the material for a magnetic flux blocking plate is desired to be such a nonmagnetic material as copper, aluminum, silver or silver alloy, or the like, which is electrically conductive so that inductive current is allowed to flow through the magnetic flux blocking plate, and also is small in specific resistance. Also, ferrite or the like, which is capable of confining magnetic flux, but is relatively high in specific resistance, is desirable as the material for a magnetic flux blocking plate. Further, magnetic material such as iron or nickel can be used as the material for the magnetic flux blocking plate, with the condition that a magnetic flux blocking plate is to be provided with through holes in the form of a circle or a slit to minimize the heat generation by eddy current.
However, in the case of the heating apparatuses according to the prior arts, the magnetic flux blocking plate is placed close to the heating medium, and therefore, they have the following flaws:
Generally, metals such as copper, silver, aluminum, or the like, are high in electrical conductivity. Thus, if the magnetic flux blocking plate is formed of copper, silver, aluminum, or the like, the amount by which heat is conducted to the magnetic flux blocking plate from the heating medium increases in proportion to the thermal capacity of the magnetic flux blocking plate, reducing thereby the rate at which the temperature of the heating medium increases. On the contrary, if the thickness of the magnetic flux blocking plate is extremely reduced to reduce the thermal capacity of the magnetic flux blocking plate, not only does the magnetic flux blocking plate fail to completely block the magnetic flux, but also heat is generated in the magnetic flux blocking plate itself due to the concentration of the magnetic flux, increasing the temperature in the adjacencies of the inductive heat generating source, which in turn destroys the insulating property of the insulating layer which covers the coil, that is, the inductive heat generating source.
When a magnetic flux blocking plate is disposed close to a cylindrical heating medium, it must be made arcuate. However, the magnetic material such as ferrite which has a large specific resistance is generally interior in formability, making it difficult to form an arcuate magnetic flux blocking plate using such magnetic material.
It is possible to form a magnetic flux blocking plate using magnetic substance such as iron, nickel, or the like, and to provide the magnetic flux blocking plate with round holes and/or slits to minimize the effects of the heat generated therein. In such a case, however, the magnetic flux reaches the heating medium, although by only a small amount, generating heat in the portion of the heating medium outside the recording medium path, creating waste in terms of energy consumption.
The primary object of the present invention is to provide a fixing apparatus capable of preventing the temperature of the portion of its heating medium outside the recording medium path from rising.
Another object of the present invention is to provide a fixing apparatus shorter in the startup time than a fixing apparatus in accordance with the prior arts.
According to an aspect of the present invention, there is provided an image fixing apparatus comprising:
magnetic field generating means for generating a magnetic flux;
a heating member generating heat by induction heating by the magnetic flux generated by said magnetic field generating means; and
a blocking plate, disposed for movement between said magnetic field generating means and said heating member, for blocking the magnetic flux from said magnetic field generating means,
wherein said blocking plate comprises an electroconductive member having a thickness of 0.1-2 mm.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.